why my van lose power at high speed

Hello, The knocking may be a bad catalytic converter element (the part that looks like honeycomb inside the converter. If it is loose, it will rattle, and it can sound pretty loud. If it is loose, it can block exhaust gases from escaping, and cause the engine to work harder to do the same amount of work or even bog down.

So many possibilities on something that old you should really take it to a professional.